Our preliminary observations on the effect of PB-induction on benzpyrene oxidation suggest that the effect of foreign chemicals on benzpyrene induced carcinogenesis will be highly strain dependent. This variability arises in the selectivity of the cytochrome P450-oxidase which is induced by PB not in a failure to induce the cytochrome. Clearly, these results are very relevant to carcinogenesis if either benzpyrene 4,5-epoxide of the 9,10 dihydrodiol is the ultimate carcinogen. We are particularly interested in investigating the microsomal oxidation of other polycyclic hydrocarbons to determine whether phenobarbital induced a specific aryl hydrocarbon K-region oxidase. The correlation between the inhibitory effects of TCPO on epoxide hydration and inhibition of the total oxidation rate implies that the latter derives in some way from the accumulation by epoxides. The effect is more pronounced with PB-induced microsomes where selectivity by the oxidases for the 4,5 bond yields a higher proportion of the more stable 4,5 epoxide. Two alternative explanations seem applicable: (a) Epoxides inhibit the oxidase process; or (b) epoxides are reduced back the substrate hydrocarbon as recently reported.